It is desirable to develop both processes and apparatus which will allow for efficient storage and retrieval of energy. Improvements in energy storage and retrieval have important applications in many different fields, such as in systems in automobiles, or the buffering of energy produced by intermittent sources (like wind turbines or solar panels) so that the energy may be accumulated, stored and then released when needed (or when the price is highest in reflection of that need).
Many different types of energy storage system are already well known. The most common of these are rechargeable electrical batteries including simple common examples like lead acid batteries in automobiles, and extending to more recent innovations in Lithium ion based and other cells. Rechargeable electrical cells are among the most widely used common energy storage/retrieval systems. In other scales and time frames—flywheels may be used to keep the rotational speed of generators or shafts constant, water may be pumped up hill to provide large scale energy storage and retrieval systems when used in conjunction with hydroelectric dams, and at least two instances exist of Compressed Air Energy Storage (CAES) where wind farms (or other intermittent sources) are used to generate electricity which is used to power compressors which pump compressed air into underground caverns where the potential energy within the compressed air remains stored. The stored energy may then be used to provide most of the drive to gas expansion turbines (like the General Electric LM2500) but because of the thermodynamics of expanding gas from 1000 psi to 15 psi (1 atmosphere) large temperature losses occur within the expanded gas, and to maintain an operational system natural gas must be burned in the gas turbines to provide adequate heat to allow for “reasonable” operating temperatures.
There are known problems with known CAES systems, and these relate to two specific areas. First, known systems are not truly “renewable” because they rely on the burning of natural gas (or some other fuel) to provide heat to balance the thermodynamics of the system. Second, they are relatively inefficient with a total efficiency of between 30 and 40 percent (where efficiency is defined as the amount of energy out divided by the amount of energy in).
Rufer et al. in WO 2008-139267 have identified the ultimate basic efficiencies possible through the use of piston compression and expansion of gas, and in particular through the use of liquid pistons to achieve this compression/expansion. Rufer et al. teach the use of a shuttling device to separate hydraulic motor pump fluid from the working fluids of the storage vessel, and about the energy densities, and efficiencies which are attainable with such an apparatus. Rufer et al. further teach that heat exchange within the “liquid piston” part of the apparatus will improve the possible energy densities. If one chooses either of two boundary conditions for the physical system responsible for the gas compression expansion—either adiabatic or isothermal, then it follows that the process itself (not realizable in a real world apparatus) could be 100 percent efficient. Rufer et al. teach, however, that a process which is quasi-isothermal will achieve much better energy storage densities per unit volume of compressed gas.
Further details are provided in the Ph. D. thesis of Sylvain LEMOFOUET—GATSI, entitled “Investigation and optimisation of hybrid electricity storage systems based on compressed air and supercapacitors.” (Thesis N 3628 (2006), Swiss Federal Institute of Technology, Lausanne (EPFL Lausanne—Switzerland).
Van de Ven and Li (Applied Energy 86, 10) teach of the high efficiencies (greater than 83 percent) obtainable with liquid piston compressors.
Kenway at el. in PCT Application Publication WO 2009-076757 teach that the thermodynamics may be better managed by limiting the gas compression ratios to approximately 3.2:1. The disclosed apparatus makes use of common commercially-available components to achieve the implementation of hydraulic-pneumatic compression.
Adler and Siebert in PCT Application Publication WO 2006-034748 further teach of the practical realizable design of a device for compressing a gaseous medium, particularly hydrogen. It is taught that by use of an appropriate liquid (an ionic liquid), it is possible to achieve very high compression (and compression ratios) since the full advantages of liquid pistons can be exploited without fear of cavitation of the drive pump(s)/motor(s).
Cavitation (or fizz) is the highly destructive appearance of bubbles formed by entrained gases and usually nucleated around small impurities in the hydraulic fluid. If the expansion of the gas is for example 1000 times, then a bubble that was entrained at a scale of 10 microns expands to 10 mm with the destructive force of a small explosion.
Adler and Siebert and Van Ven and Li further teach that the liquid pistons easily accommodate heat exchangers in the compression chamber (or cylinder) so that maintaining quasi-isothermal conditions is much more easily achieved than with conventional compressors or expanders.
The following references are also of use for understanding the state of the art: U.S. Pat. No. 3,947,736 (Byers et al.); U.S. Pat. No. 4,286,203 (Ehret); U.S. Pat. No. 3,971,972 (Stitch); U.S. Pat. No. 4,128,793 (Stitch); U.S. Pat. No. 4,618,810 (Hagerman et al.); U.S. Pat. No. 4,364,073 (Becke et al.); Bose, Bimal K. (1980). Adjustable Speed AC Drive Systems. New York: IEEE Press. ISBN 0-87942-146-0; Heinlein, R. (1982). Friday. New York, Holt Reinhart and Winston-Shipstone